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Sweltering Summer Lessons
Growers in a North American setting exposed to the hot outdoor weather of summertime have a high chance of encountering one of the most pernicious, destructive pests in Cannabis: The Budworm. They start their lives as nigh-imperceptible eggs laid on flowers at night by moths traveling hundreds of kilometers with as many as a thousand eggs per female. Early after hatching, these tiny larvae—2 millimeters in length—burrow into the richly aromatic buds, totally unabated by these volatiles which often serve as defense compounds against other pests. As they chew through, their bodies engorge with plant tissue and excrete small solid black fecal pellets that, combined with the resulting wounds, create a hotbed for opportunistic pathogenic growth. Swaths of premium product are consumed in this process. One larva can foul a significant amount of an individual plant’s yield by itself and is usually accompanied by Botrytis Budrot exacerbating further. After growing about nine times in length, the 18-millimeter-long larva extracts from its food and convenient shelter to look for soil in which to burrow and pupate, emerging as an adult moth soon after in warm months and overwintering in the cold. Budworm pupae survive in the ground in places where it doesn’t get too cold, typically south of 40° latitude.
Budworms are difficult because they almost exclusively eat floral tissue, robustly resist plant defenses, and cannot be easily reached with treatments safe for flower-like botanical contact-killing compounds or microbes because they tunnel deeply. Even if they are killed, the caterpillar cadaver and waste will fester and mold, so this factor must be managed even in the most successful reactive treatments.
There are actually multiple closely related moth species that, as larvae eat Cannabis flowers, along with hundreds of other crop and uncultivated plant species, provide enough food for populations to swell. The most represented species are the Corn Earworm, Helicoverpa zea, and Cotton Bollworm, Helicoverpa armigera, the latter of which is also found extensively in Eurasia and South America, where it is estimated to cause around 3 billion dollars in agricultural damage annually, especially in food crops. Unfortunately, the species are difficult and sometimes impossible to confidently differentiate, especially since there is evidence of hybridization and increased adaptive performance from overlapping expansion.
With the precipitous increase of Cannabis cultivation globally, outdoor cultivation for medicinal, culinary, and textile products has come under stronger pest pressure as both new and historical locations lack much of the infrastructural support on which other agricultural crops rely for technical research and pest treatment knowledge, not to mention basic legislative acceptance at several levels. This austere context has made budworms and other pests like Hop Latent Viroid challenging to contain for the Cannabis cultivator, whether residential or commercial.
Defeating The Very Munchy Caterpillar
Preventative measures that rely on disrupting adult egg laying can be the most cost-effective solution. By exploiting this vulnerability with physical barriers that don’t allow contact between the flowers and the moth’s abdomen, larval presence is avoided. Insect netting is often sufficient and may also repel or disrupt the colonization of other pests, especially if the openings are fine such as those that are marketed for thrips between 4-8 microns. Staging such barrier screens is as important as the screen itself: If there are large gaps or tears that allow the primary target to ingress, the barrier becomes significantly less effective. In some areas where humidity levels can spike, fine screens can reduce ventilation which might facilitate microbial growth like that of Botrytis or Powdery Mildew. Movement between barriers meant for these moths must be simple and support cultivation techniques carried out between the onset of flowering like the preventative application of caterpillar-killer microbes. Integrating these factors in a streamlined procedure is contextual and will look different between settings.
Removing non-crop plants on property suitable as hosts for pests decreases their proximity to Cannabis. Regularly looking for evidence of pest presence helps but learning the common hosts of specific species is more valuable, and this too is true of the budworm. Budworm moths are nocturnal and actively lay eggs at night. While they can travel great distances, it may take more than one generation to establish closely, and taking advantage of their off-season inactivity as pupae in the colder months before and at the start of spring to clear hosts and acquire supplies maintains sharp readiness. Agricultural crops like tobacco, cotton, cabbage, and tomato are some of the most well-documented; cultivating near these plants is likely to be more difficult, especially if they are in large groups or very close. There are many places where feral Cannabis “ditch weed” grows profusely, which could also have a concentrating effect.
Pest research specific to Cannabis has steadily increased in the past few years for certain targets. Prominent scholastic suggestions for budworm treatment emphasize the importance of seasonal trapping using special attractant pheromones that lure males to evaluate how many and at what time these specific moths are generally active in the area. Changes in population will differ year to year, and the moths can have multiple generations in many places, so setting these traps to track when the first moths appear and then start to rise informs the best time to apply microbial agents to manage any larvae that present. In North America, individuals can be active earlier but spike sometime between July and August.
Two main microbes are applied for budworm larvae: the bacteria Bacillus thuringiensis, usually shortened to “Bt,” and the Helicoverpa zea Nuclear Polyhedrosis Virus, shortened to HearNPV, which is a natural specialist of only this budworm lineage. In either case, these microbes must be in contact with the floral tissue that is ingested to be effective. Younger larvae are the most vulnerable to these pathogens and become progressively resistant as they age, still killing or possibly maiming older stages through sublethal effects that commonly prevent development to adulthood. Ideally, the first larval stage can be killed before it starts tunneling or very soon after when it can only eat a small portion and cause the most minimal damage. Important to note that since harvest season at or near autumn can see elevated numbers, post-harvest inspection is crucial to reduce the number of caterpillars that might be active as well as the removal of wounded or fungal sites if present. The quicker harvested flower can be dried, the faster it becomes unpalatable to the larvae as well. Together with comprehensive barrier placement, pheromone trapping, and a keen eye, microbial biocontrol agents are an excellent additional layer of protection when applied strategically.
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Photo Credit: Corn Earworm September 2018 Update, University of Colorado